In view of the high sintering temperature and difficulty in densification of the ultra-high temperature ceramics(UHTCs), a novel reactive hot-pressing process was designed to achieve the low temperature densification of high strength (Zr,Ta)B 2 -SiC ceramics. It was found that the mechanical properties were closely associated with the microstructure, being dependent on the sintering temperature. The microstructure characterization revealed that the main phases of ceramic were incompletely solid soluted borides and SiC at the sintering temperature of 1600 °C; while at 1700 °C, the boride ceramics were completely solid soluted (Zr,Ta)B 2 and formed an interlocking structure with SiC. The evaluation of mechanical properties indicated that the flexural strength of the ceramics increased from 470.2 MPa to 736.7 MPa when the sintering temperature increased from 1600 °C to 1800 °C. Correspondingly, the fracture toughness increased from 3.54 MPa·m 1/2 to 4.10 MPa·m 1/2 . The improvement of mechanical properties were mainly attributed to the interlocking structure formed by the solid solution (Zr,Ta)B 2 and SiC, which caused the crack propagation path becoming tortuous along the interface of different phases.
(Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )B 2 -SiC ceramics with an open porosity of only 0.49 % were successfully fabricated in this work by the reactive melt infiltration (RMI) method. Air plasma flame ablation behavior of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )B 2 -SiC ceramics under 2300 °C temperature exhibited excellent resistance with mass and linear ablation rates of 3.66 mg/s and 0.88 μm/s, respectively. During ablation, the transition from continuous melting sublimation of Si and oxidation of SiC to oxidation of (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )B 2 occurred. The uphill diffusion of Ti atoms transited (Ti,Zr,Hf,Nb,Ta)O x to Ti(Nb x Ta 1-x ) 2 O 7 while Zr 0.5 Hf 0.5 O 2 continuously precipitated. A multicomponent oxide layer of (Ti,Zr,Hf,Nb,Ta)O x along with (Zr,Hf) 6 Ta 2 O 17 , (Hf,Zr)(TiO 4 ) 2 , and (Zr,Hf) x (Ti,Ta) 1-x O embedded in SiO 2 melt was eventually formed on the ceramic surface. This oxide layer was spread above the high melting point Zr 0.5 Hf 0.5 O 2 , Ti(Nb x Ta 1-x ) 2 O, and low oxygen content (Ti,Zr,Hf,Nb,Ta)O x to form a stable, dense, and high viscosity protective layer.
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